The present invention relates broadly to an alarm system for monitoring the presence or absence of alarm conditions at a remote location. In particular, the present invention is an alarm system that includes a transmitter positioned at a remote location and connected to a receiver over conventional power or communication lines. The transmitter generates a signal representative of the presence of absence of an alarm condition and the generated signal is received and processed at the monitoring location by the receiver.
It is often desirable to monitor conditions at remote locations for detection of conditions that require attention. For example, in agricultural applications it is often desirable to monitor a plurality of conditions within lifestock confinement buildings. Such buildings are typically located some distance from the dwelling or residence of the cattleman or farmer and it is imperative that he be alerted upon occurrence of an abnormal condition within the livestock confinement area. In addition to fire detection, the cattleman or farmer may also want to monitor temperature, water pressure, presence or absence of power, and the opening of the door or windows of the confinement building. It is also desirable that the alarm at the monitoring location not be triggered in response to spurious nuisance alarm signals, such as a brief power failure. Additionally, it is also desirable to have an alarm system which will also be triggered upon loss of power at the monitoring location. If such a feature is not provided, alarm conditions at the monitored location may go undetected for a significant period of time resulting in extensive damage to the livestock. For example, if the temperature in the confinement building were allowed to increase above the desirable range without detection, the livestock may be subjected to severe stress and, perhaps, even death.
Alarm systems which have receivers and transmitters connected over conventional power lines are known in the prior art. Typically such prior art systems include continuous wave transmission at a predetermined frequency. Such systems are particularly susceptible to AC line noise. One prior art system also incorporates amplitude modulation in the transmission and detection circuitry. Amplitude modulated signals, however, are more prone to noise interference than frequency modulated systems. Additionally, the prior art systems have no provision for monitoring loss of power at the receiver or monitoring location.
The present invention eliminates the disadvantages of the prior art alarm systems in that it incorporates frequency shift keying in a substantially fail-safe system which generates an alarm in response to the detection of an alarm condition at the transmitter location and also generates an alarm in response to power failure at either the transmitter or receiver. The alarm system of the present invention operates with a modulated radio frequency signal connected over conventional power lines between the receiver and transmitter. The alarm system is therefore essentially immume from AC line noise. The alarm system of the present invention also includes time delay circuitry that functions to generate a valid alarm indication only after a predetermined time delay following detection of an alarm condition or power failure. Nuisance alarms are thereby eliminated in the present invention. The alarm system of the present invention is readily adaptable for monitoring a plurality of conditions at a remote location. Each such condition is provided with independent receiver and transmitter circuitry operating on different nominal frequencies or channels.